1. Field of the Invention
The present invention relates to a speed change gearing and, more particularly, a friction speed change gearing which transmits a rotational power while effecting a change in the rotational speed by employing a planetary mechanism including an inner race, an outer race and a plurality of cylindrical rollers interposed therebetween, wherein the rotational power is transmitted by friction engagement.
2. Description of the Prior Art
The friction speed change gearing having the aforementioned basic structure is known and is, in fact, practically used in accordance with various particular designs. In conventional designs, the friction speed change gearing of this particular type employs a spider member having spider fingers each of which is engaged into the planetary roller to operate as a bearing shaft which rotatably supports the roller along its central axis so that the rotary power is transmitted from the planetary rollers to the spider member or vice versa when the planetary rollers make a planetary movement along an annular space defined between an outer race and an inner race. However, this conventional design has a drawback in that when the rollers rotate at a high speed, a large quantity of heat generated in the roller is accumulated in the bearing portion and causes heavy wearing or sometimes burning or sticking. Furthermore, since the cylindrical planetary rollers are mounted between the inner and outer races and are under or subjected to an elastic deformation, a desired amount of power is transmitted among these frictionally engaged members depending upon the theory of elastic fluid lubrication, the cylindrical roller always rotates in a somewhat elliptically deformed condition, and, for this reason, it is difficult to provide a rotary bearing for the roller which can support it constantly in a smooth rotary engagement.
In view of these problems, in Japanese Patent Application No. 49793/73 the assignee has proposed to build a friction speed change gearing of this type by employing a spider member which engages among adjacent planetary rollers in a manner such that it slidably contacts the outer surface of the rollers instead of rotatably supporting the rollers at their axial portions. This new type of friction speed change gearing obviates the aforementioned problems involved in the conventional design of the friction speed change gearing and, when the bearing surface of the spider member which slidably contacts the outer surface of the roller is properly designed in a shape such that it generates a lubricating oil film between the spider member and the planetary roller, it certainly provides an improved performance. However, if we note the fact that in this type of friction speed change gearing the engagement between the inner and outer races and the planetary rollers is all rotary contact, and that due to this rotary contact this type of friction speed change gearing provides a very high transmission efficiency or, in other words, a very low friction loss, it will be appreciated that the sliding contact between the planetary rollers and spider member which is the sole sliding contact existing in this structure has a very important effect on the overall efficiency of the gearing and that a further improvement with regard to this point is very worthwhile for accomplishing further improvements with respect to this kind of friction speed change gearing.